Beam riding is a known technique for guiding a missile to its target. In this technique, a laser beam coded in azimuth and elevation is projected towards the target, and the missile is provided with light sensors for detecting the beam. Once launched, the missile uses the sensors to correct its position to a specific location within the beam, allowing it to travel along the path of the beam towards the target.
FIG. 1 shows an example of a missile 101 being guided towards a target 103 by a laser beam riding missile guidance system. In this example, the target 103 is an aircraft, but could also be a ground based target such as a tank or a sea-based vessel.
The system comprises a laser operable to generate an intermittently projected laser beam 105. An operator uses an optical sight to align the beam with the target. The laser is scanned in lateral and vertical directions with respect to the direction in which the beam is propagating, so as to form a laser information field 107. The laser information field comprises an array of points or grid, in which the light signal at each point is modulated with information that can be used to identify that point's position within the array.
The missile 101 is provided with aft mounted sensors that can detect the signal encoded in the laser beam and so determine the missile's position with respect to the centre of the laser information field. Then, by use of appropriate guidance mechanisms (e.g. fins), the missile can adjust its position so as to remain aligned with the centre of the beam.
As shown in FIG. 1, a problem that may arise in the beam riding missile guidance system is that an optical alignment error 109 exists between the centre of the laser information field and the target aimpoint centre 111 in the optical sight. The misalignment can lead to a guidance error which can result in the missile missing the target.